Surgeons use ultrasonic instruments in surgery to cut and coagulate tissue. Piezoelectric elements are electrically excited at a resonant frequency of an ultrasonic instrument to create vibrations that are transmitted through a resonator and amplified to produce a mechanical, standing wave vibration of the same frequency. An ultrasonic transmission assembly of the instrument has an elongated, transmission waveguide that transmits this vibration to an end effector (e.g., cutting blade) on the distal tip of the instrument. The end effector may vibrate primarily in the longitudinal direction to generate localized heat within adjacent tissue, although some instruments have been designed specifically so that the end effector vibrates primarily in either of the transverse (perpendicular to the longitudinal axis) or torsional (about the longitudinal axis) directions to treat tissue.
The distal tip of the end effector corresponds to a vibratory anti-nodal point. The proximal end of the end effector typically attaches to the waveguide slightly distal to the most distal, vibratory nodal point of the ultrasonic transmission assembly. This arrangement allows tuning of the instrument to a preferred resonant frequency when the end effector is not loaded with tissue. In some embodiments, the length of the end effector is slightly less than one-quarter of the acoustic wavelength that propagates through the end effector material when excited by an ultrasonic energy input of a particular frequency.
Ultrasonic surgical end effectors formed from different materials may exhibit significantly different acoustical and mechanical characteristics. These characteristics may be associated with material properties such as ultrasonic propagation wavelength, conductive heat transfer, mechanical fatigue strength and acoustic transmission efficiency. For example, an end effector formed from a material such as a ceramic having a relatively high ratio of elastic modulus to density may have a longer ultrasonic propagation wavelength than that of an end effector formed from a material such as a metal having a relatively low ratio.
End effectors of some current ultrasonic surgical instruments are made of a Ti-6Al-4V titanium alloy. The ultrasonic propagation wavelength of the titanium alloy is about 87 mm when operated at an ultrasonic frequency of 55.5 kHz, so that the length of the end effector is about 22 mm. For certain surgical applications the surgeon may prefer a slightly longer end effector than what is currently available.
The acoustic wavelength in a material is equal to the speed of sound in the material divided by the frequency (cycles/sec.) of the ultrasonic energy input. Therefore, one way to provide instruments with longer end effectors is to decrease the frequency of the ultrasonic energy input. For example, reducing the frequency from approximately 55.5 kHz to approximately 27.8 kHz increases the characteristic wavelength in a titanium alloy to approximately 174 mm. However, there is a practical lower limit to excitation frequency. An end effector vibrating below 20 kHz may create a painfully audible sound to humans and obviously would not be desirable in a surgical operating room.
Another way to provide instruments with longer end effectors is to select end effector materials in which sound travels faster. The speed of sound in a material is a function of material density and modulus of elasticity. Basically, materials having a high elastic modulus to density ratio propagate ultrasonic energy faster than materials having a relatively low ratio. Certain ceramic materials, including alumina (Al2O3), exhibit characteristic wavelengths that are approximately twice as great as some titanium alloys. Unfortunately, ceramic materials are very brittle and ceramic end effectors would be susceptible to breakage during normal handling, set-up and operation.
In addition to providing longer end effectors, it may be desired to improve the acoustical transmission efficiency of the end effector in order to reduce “self-heating” of the end effector and the time to cut and coagulate tissue. Some materials such as sapphire, titanium and aluminum may transmit ultrasonic energy more efficiently than other materials such as copper and steel. Acoustical transmission efficiency of surgical ultrasonic end effectors may be associated with a unitless acoustical coefficient, known in the art as the “Q” coefficient, which for the Ti-6Al-4V titanium alloy and some aluminum alloys is in the range of 10,000 to 20,000. The Q coefficient for certain steels may be as low as 250. For applications in which self-heating of the end effector should be minimized, the end effector may be formed from a material having a high Q coefficient. However, there may be some surgical applications in which rapid self-heating of the end effector is desired, such as when the end effector is used while immersed in body fluids. For such applications, the end effector may be formed from a material having a lower Q coefficient in order to quickly generate heat in the tissue to cut and coagulate the tissue.
The thermal conductivity of the end effector material may also significantly affect how quickly the end effector cuts and coagulates tissue. If the end effector conducts heat to the tissue too quickly, the tissue may char. But if the end effector conducts heat to the tissue too slowly, the device may cut and/or coagulate too slowly. Depending on the surgical application, an end effector formed from the Ti-6Al-4V alloy, which has a thermal conductivity of about 7 W/m-K, may retain too much heat, whereas an end effector formed from aluminum, which has a thermal conductivity of about 200 W/m-K, may pull too much heat away from the tissue.
The mechanical fatigue strength of the end effector material may significantly affect the operational life of the end effector and, consequently, how many times the end effector can be used during a surgical procedure. Fatigue strength is sometimes referred to as the endurance limit of the material and corresponds to the stress at which the material may be reversibly stressed for practically an infinite number of cycles. The Ti-6Al-4V alloy has a fatigue strength of about 413 kPa, whereas the fatigue strength of aluminum is about 138 kPa. Aluminum also is softer than the titanium alloy and is more easily damaged by other surgical instruments during usage, possibly leading to crack initiation that may further reduce the fatigue resistance of the end effector.
Clearly, the design of surgical ultrasonic end effectors has been very challenging at least in part because the available choices for a single end effector material that has the combination of acoustical and mechanical characteristics desired for certain surgical applications is very limited. For example, it may be desired to provide a surgical ultrasonic end effector that has a longer ultrasonic propagation wavelength and a greater fatigue strength than current end effectors, and yet maintains the acoustic efficiency and thermal characteristics of current end effectors.
Another surgical instrument is disclosed by U.S. Pat. No. 6,375,635 which uses a liquid jet for cutting tissue. The instrument includes a pressure lumen that conducts a high pressure liquid towards a distal end of the instrument and that includes a nozzle that provides a jet opening. The instrument further includes an evacuation lumen opposite the jet opening to receive a liquid jet when the instrument is in operation.